Foil for negative electrode of capacitor and process for producing the same

ABSTRACT

Provided are a foil for a cathode of a capacitor, which can secure both a high capacitance and a high strength, and a manufacturing method thereof. The foil for a cathode of a capacitor includes an aluminum foil and a carbon-containing layer formed on a surface of the aluminum foil. An interposition layer containing aluminum and carbon is formed between the aluminum foil and the carbon-containing layer. The manufacturing method of a foil for a cathode of a capacitor includes a step of arranging an aluminum foil in a space containing a hydrocarbon-containing substance and a step of heating the aluminum foil.

TECHNICAL FIELD

This invention relates to a foil for a cathode of a capacitor, and amanufacturing method thereof. More particularly, this invention relatesto a foil for a cathode of a capacitor, which uses an aluminum foil as abase material, and a manufacturing method thereof

BACKGROUND ART

A capacitor includes two electrodes: an anode and a cathode. As amaterial of the anode, there is used a valve metal such as aluminum ortantalum which allows an insulating oxide film to be generated on asurface. As a material of the cathode, there is used any of anelectrolytic solution, an inorganic semiconductor, an organic conductivematerial and a metal thin film. When the material of the cathode is anelectrolytic solution, an aluminum foil having an enlarged surface areais frequently used for a cathode terminal. An aluminum foil of this typeis called an electrolytic capacitor cathode aluminum foil. The surfacearea of the cathode aluminum foil is enlarged to increase thecapacitance of a capacitor.

A manufacturing process of a capacitor involves a step of cutting acathode aluminum foil supplied in the form of a coil into apredetermined width, a step of coiling the slit coil-shaped cathodealuminum foil which has been cut, and a step of coiling the cathodealuminum foil in order to finally constitute a cylindrical capacitor ora step of cutting the cathode aluminum foil in order to constitute alaminate-type capacitor.

In order to improve productivity in these steps, it is required to raisethe rate of coiling the cathode aluminum foil. To meet this demand, itis necessary to improve the mechanical strength of the cathode aluminumfoil, that is, the tensile strength of the cathode aluminum foil.

As mentioned above, the cathode aluminum foil requires a high tensilestrength as well as a high capacitance as necessary requirements.

For increasing the capacitance of the cathode aluminum foil, there isgenerally adopted a method of enlarging the surface area of the aluminumfoil by etching. The etching, however, significantly reduces the tensilestrength of the aluminum foil.

There is a current need for developing a smaller capacitor along withthe development of small-sized electric apparatuses. To make a capacitorsmall, the thickness of the cathode aluminum foil must be made moresmall. If the thickness of the aluminum foil is made thin, the strengthof the aluminum foil is relatively dropped.

Also, as the capacitor cathode aluminum foil, one having an aluminumpurity of about 99.0 to 99.95% by mass is usually used to control thereaction when carrying out etching to enlarge the surface area of thealuminum foil. The purity (base purity) of aluminum herein means a valueobtained by subtracting the content of major three elements, i.e., iron,silicon and copper, included in the aluminum foil from 100%.

In this situation, there is proposed a manufacturing method forimproving the capacitance and strength of an electrolytic capacitorcathode aluminum foil in, for example, Japanese Unexamined PatentPublication No. 05-247609. In this manufacturing method of theelectrolytic capacitor cathode aluminum foil, an aluminum materialhaving a chemical composition in which the purity of aluminum is 99.8%or more, the contents of Fe and Si are respectively limited to 0.05% orless and the content of Cu is limited to 0.005% or less and the contentof Mg and the content of Zn are adjusted so as to satisfy apredetermined equation, is subjected to soaking treatment at 540° C. orless, hot-rolled and then subjected to coiling which is finished at atemperature of 300° C. or less, and is finally subjected to cold rollingcarried out at a working ratio of 95% or more to make the product foilhave the specified thickness. This aluminum foil is etched to enlargethe surface area.

However, even if the foregoing manufacturing method disclosed inJapanese Unexamined Patent Publication No. 05-247609 is used, it islimited in its capability of improving the capacitance of a capacitorwithout any reduction in tensile strength as to the requirementssatisfied by the capacitor cathode aluminum foil.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide a foil for a cathode of acapacitor, that can secure a high capacitance and a high strength, and amanufacturing method thereof.

The inventors of this invention have made earnest studies to solveproblems about the background art and, as a result, have found that afoil for a cathode of a capacitor, that can attain the above object, canbe obtained by subjecting an aluminum foil to specific treatment. Thepresent invention has been thus completed based on the findings of theinventors.

A foil for a cathode of a capacitor according to this invention includesan aluminum foil and a carbon-containing layer formed on the surface ofthe aluminum foil. An interposition layer containing aluminum and carbonis formed between the aluminum foil and the carbon-containing layer.

In the foil for a cathode of a capacitor according to this invention,the carbon-containing layer has the effect of enlarging or increasingthe surface area of the aluminum foil without any reduction in thestrength of the aluminum foil. This increases the capacitance of thefoil for a cathode of a capacitor. In addition, since the interpositionlayer containing aluminum and carbon is formed between the aluminum foiland the carbon-containing layer, this interposition layer has the effectof improving the adhesion between the aluminum foil and thecarbon-containing layer that increases the surface area of the aluminumfoil. A predetermined capacitance and a predetermined strength can bethereby secured.

In the foil for a cathode of a capacitor according to this invention,preferably, the carbon-containing layer includes therein aninterposition material containing aluminum and carbon.

When the carbon-containing layer is thin, the adhesion between thealuminum foil and the carbon-containing layer can be improved only bythe presence of the interposition layer. However, when thecarbon-containing layer is thick, separation inside thecarbon-containing layer occurs; therefore, there is a possibility that apredetermined capacitance cannot be obtained. In this case, if theinterposition material containing aluminum and carbon is formed insidethe carbon-containing layer, the adhesion in the carbon-containing layercan be raised, making it possible to secure a predetermined capacitance.

In the foil for a cathode of a capacitor according to this invention,preferably, the carbon-containing layer is formed so as to extendoutward from the surface of the aluminum foil. In this case, thecarbon-containing layer exhibits the effect of enlarging or increasingthe surface area of the aluminum foil more effectively.

In the foil for a cathode of a capacitor according to this invention,preferably, the interposition layer constitutes a first surface portionwhich is formed in at least a part of the surface of the aluminum foiland contains a carbide of aluminum. Also preferably, thecarbon-containing layer constitutes a second surface portion which isformed so as to extend outward from the first surface portion.

In this case, the second surface portion has the effect of increasingthe surface area of the aluminum foil without any reduction in thestrength of the aluminum foil. The capacitance of the foil for a cathodeof a capacitor is thereby increased. Further, since the first surfaceportion containing a carbide of aluminum is formed between the aluminumfoil and the second surface portion, this first portion has the effectof improving the adhesion between the aluminum foil and the secondsurface portion that increases the surface area of the aluminum foil. Itis thereby possible to secure a predetermined capacitance and apredetermined strength.

A manufacturing method of a foil for a cathode of a capacitor accordingto this invention includes a step of arranging an aluminum foil in aspace containing a hydrocarbon-containing substance, and a step ofheating the aluminum foil.

In the manufacturing method according to this invention, aninterposition material containing aluminum and carbon can be formed anda carbon-containing layer having the effect of increasing the surfacearea of the aluminum foil can be easily formed on the surface of thealuminum foil by heating the aluminum foil arranged in a spacecontaining a hydrocarbon-containing substance.

In the manufacturing method of a foil for a cathode of a capacitoraccording to this invention, preferably, the step of disposing thealuminum foil involves adhering at least one kind selected from thegroup consisting of a carbon-containing substance and an aluminum powderto a surface of the aluminum foil and, then, arranging the aluminum foilin a space containing a hydrocarbon-containing substance.

When the aluminum foil arranged in a space containing ahydrocarbon-containing substance is heated, a foil for a cathode of acapacitor provided with a capacitance and a strength higher than that ofa conventional one can be obtained. However, in the case of obtaining afoil for a cathode of a capacitor having an outstandingly highercapacitance, it is preferable to adhere at least one kind selected fromthe group consisting of a carbon-containing substance and an aluminumpowder to a surface of the aluminum foil, then, arrange the aluminumfoil in a space containing a hydrocarbon-containing substance, and heatthe aluminum foil.

It is preferable that the manufacturing method of a foil for a cathodeof a capacitor according to this invention further includes a step ofrolling the heated aluminum foil.

In the manufacturing method of a foil for a cathode of a capacitoraccording to this invention, the step of heating the aluminum foil ispreferably carried out within a temperature range between 450° C. ormore and less than 660° C.

According to this invention, as mentioned above, it is possible toobtain a foil for a cathode of a capacitor capable of securing a highcapacitance and a high strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a scanning-type electron microphotograph of a sample ofExample 1.

FIG. 2 shows a scanning-type electron microphotograph of a sample ofExample 3.

FIG. 3 shows a scanning-type electron microphotograph of a sample ofExample 6.

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors of this invention have found that when an aluminummaterial is heated in the presence of a hydrocarbon-containingsubstance, a crystal of aluminum carbide is formed as a first surfaceportion in the vicinity of the surface of the aluminum material and asecond surface portion is formed at a high density so as to extendoutward the surface of the aluminum material from the crystal ofaluminum carbide. The inventors of this invention have also found thatheating of the aluminum material brings about the characteristics thatthe formed second surface portion is firmly bound internally or withother carbon-containing substance firmly to form an aluminum foil havinga larger surface area. The present invention was thus completed.

The aluminum material produces the above effect in any of a plate shape,a foil shape and a powder shape; however, an aluminum material having afoil shape is used for a cathode of a capacitor.

The second surface portion according to one embodiment of this inventionextends outward from the first surface portion containing a carbide ofaluminum, and is preferably present in a filament form, a fibrous formor a cactus form so that it has a large surface area, which has theeffect of increasing the surface area of the aluminum foil. As a result,the capacitance of the foil for a cathode of a capacitor is increased.

In one embodiment of this invention, the first surface portioncontaining a crystal of aluminum carbide is formed between the aluminumfoil and the second surface portion; therefore, this first portion hasthe effect of improving the adhesion between the aluminum foil and thesecond surface portion that increases the surface area of the aluminumfoil. This ensures that a predetermined capacitance and a predeterminedstrength can be secured.

In one embodiment of this invention, the second surface portion isinternally bound firmly; therefore, it is possible to obtain an aluminumfoil having a larger surface area by utilizing an aluminum powder.

Furthermore, in one embodiment of this invention, the second surfaceportion is firmly bound with a carbon-containing substance differenttherefrom; therefore, it is possible to obtain an aluminum foil having alarger surface area by utilizing a carbon-containing substance having alarge surface area such as activated carbon.

In one embodiment of this invention, the second surface portion isfirmly bound internally or with a carbon-containing substance differenttherefrom; therefore, it is possible to obtain an aluminum foil having alarger surface area by utilizing an aluminum powder and acarbon-containing substance.

In the case of the foil for a cathode of a capacitor according to thisinvention, any method of increasing or enlarging the surface area byetching treatment is not adopted; therefore, it is unnecessary to limitthe purity of the aluminum foil to about 99.0 to 99.5% by mass tocontrol reactivity with an etchant. As to the thickness of the aluminumfoil, an aluminum foil having a thickness of about 1 to 200 μm may beused though the thickness may be properly determined based on thestrength required for the aluminum foil and the necessity for developinga small-sized capacitor.

In one embodiment of the manufacturing method of a foil for a cathode ofa capacitor according to this invention, no particular imitation isimposed on the kind of a hydrocarbon-containing substance to be used.Examples of the kind of a hydrocarbon-containing substance includeparaffin-type hydrocarbons such as methane, ethane, propane, n-butane,isobutane and pentane, olefin-type hydrocarbons such as ethylene,propylene, butene and butadiene, acetylene-type hydrocarbons such asacetylene or derivatives of these hydrocarbons. Among thesehydrocarbons, paraffin-type hydrocarbons such as methane, ethane andpropane are gasified in the step of heating the aluminum foil and aretherefore preferable. Any one of hydrocarbons such as methane, ethaneand propane is more preferable. Methane in the hydrocarbons is mostpreferable.

In addition, the hydrocarbon-containing substance may be used in any ofa solid state, a liquid state and a gas state in the manufacturingmethod according to this invention. It is only required for thehydrocarbon-containing substance to be present in a space where thealuminum foil is present and any method may be used to introduce thehydrocarbon-containing substance into a space where the aluminum foil isarranged. When the hydrocarbon-containing substance has a gas state(e.g., methane, ethane or propane), it may be filled either singly or incombination with an inert gas in a sealed space where the aluminum foilis treated under heating. Also, when the hydrocarbon-containingsubstance has a solid or liquid state, it may be filled either singly orin combination with inert gas in a manner that it is gasified in asealed space.

There is no particular limitation to the pressure in a heatingatmosphere in the step of heating the aluminum foil and the heating maybe carried out under normal pressure or reduced pressure or underpressure. Also, the pressure may be adjusted at any time when theatmosphere is kept at a constant temperature, when the atmosphere israised to a prescribed temperature and when the atmosphere is droppedfrom a prescribed temperature.

Although no particular limitation is imposed on the weight ratio of thehydrocarbon-containing substance to be introduced in a space where thealuminum foil is arranged, the weight ratio is preferably within a rangebetween 0.1 parts by weight or more and 50 parts by weight or less,particularly preferably between 0.5 parts by weight or more and 30 partsby weight or less, in terms of carbon with respect to 100 parts byweight of the aluminum foil.

In the step of heating the aluminum foil, the heating temperature ispreferably within a range between 450° C. or more and less than 660° C.,more preferably between 500° C. or more and 630° C. or less, still morepreferably between 570° C. or more and 630° C. or less, though it may beproperly set in accordance with the composition of the aluminum foil tobe heated. However, the manufacturing method according to this inventiondoes not exclude the case where the aluminum foil is heated at atemperature less than 450° C. and it is only required for the aluminumfoil to be heated at a temperature at least exceeding 300° C.

The heating time is, though depending on, for example, heatingtemperature, generally within a range between 1 hour or more and 100hours or less.

When the heating temperature is 400° C. or more during the courseincluding a temperature-rise step and a temperature-drop step, theconcentration of oxygen in a heating atmosphere is preferably made to be1.0% by volume or less. When the heating temperature is 400° C. or moreand the concentration of oxygen in a heating atmosphere exceeds 1.0% byvolume, a thermally oxidized film on the surface of the aluminum foil ismade large and there is therefore a fear that the boundary electricresistance at the surface of the aluminum foil is increased.

Also, the surface of the aluminum foil may be roughened prior to heattreatment. There is no particular limitation to the surface rougingmethod and known techniques such as washing, etching and blasting may beused.

The aluminum foil for a cathode of a capacitor may be rolled after heattreatment so as to have a desired thickness and hardness.

In order to more increase or enlarge the surface area of the aluminumfoil by using the manufacturing-method according to the presentinvention, a step is adopted in which at least one of acarbon-containing substance and an aluminum powder is adhered to thesurface of the aluminum foil and, then, the aluminum foil is arranged ina space containing a hydrocarbon-containing substance. At this time, thesecond surface portion to be formed on the surface of the aluminum foilis constituted of a portion which extends outward from the first surfaceportion containing a carbide of aluminum and is present in a filamentform, a fibrous form or a cactus form, and a large number of particleportions adhered onto the above portion.

In this case, as the carbon-containing substance, any of an activecarbon fiber, active carbon cloth, active carbon felt, active carbonpowder, Indian ink, carbon black and graphite may be used. At to theadhering method, the carbon-containing substance prepared in the form ofa slurry, a liquid or a solid by using a binder, a solvent or water maybe adhered onto the surface of the aluminum foil by application, dippingor thermal and pressure bonding. After the carbon-containing substanceis adhered onto the surface of the aluminum foil, it may be dried at atemperature within a range between 20° C. or more and 300° C. or less.

Also, a step of arranging the aluminum foil in a space containing ahydrocarbon-containing substance is adopted after a carbon-containingsubstance is adhered onto the surface of the aluminum foil, and thenactivating treatment for increasing the surface area of thecarbon-containing substance adhered to the aluminum foil may be carriedout. The activating treatment may be carried out as follows: at atemperature within a range between 50° C. or more and 600° C. or less,preferably between 200° C. or more and 500° C. or less, the aluminumfoil is arranged in a space containing 2 to 50% by volume of oxygen. Theactivating treatment time is, though depending on the temperature,generally within a range between 10 seconds or more and 100 hours orless. This activating treatment may be repeated.

As the aluminum powder, any of an aluminum powder having an amorphous,spherical or flat form or an aluminum paste may be used. As to a methodof adhering the aluminum powder, the aluminum powder prepared in theform of a slurry, liquid or solid by using a binder or a solvent may beadhered onto the surface of the aluminum foil by application, dipping orthermal and pressure bonding.

EXAMPLES

Foils for a cathode of a capacitor were fabricated in the followingExamples 1 to 10 and a conventional example. For comparison withExamples 1 to 10, a foil for a cathode of a capacitor according to areference example was fabricated.

Example 1

An aluminum foil having a thickness of 30 μm (JIS A1050-H18) was kept at590° C. for 10 hours in an acetylene gas atmosphere. Then, when thesurface of the sample was observed by a scanning electron microscope(SEM), the existence of a carbon-containing layer was confirmed, thecarbon-containing layer being constituted of a portion extending outwardin a fibrous form or a filament form about 1000 nm in length from thesurface of the aluminum foil. This scanning electron microphotograph isshown in FIG. 1. Also, the presence of aluminum carbide was confirmed byX-ray analysis and an electron energy loss spectrometer (EELS).

Example 2

2 Parts by weight of carbon black having an average particle diameter of0.5 μm was mixed with 1 part by weight of a vinyl chloride binder andthe mixture was dispersed in a solvent (toluene) to obtain a coatingsolution having a solid content of 30%. This coating solution wasapplied to both surfaces of an aluminum foil having a thickness of 30 μm(JIS A1050-H18) and dried. The thickness of the dried coating film onone surface was 1 μm. This aluminum foil was kept at 590° C. for 10hours in a methane gas atmosphere. Then, when the surface of the samplewas observed by a scanning electron microscope (SEM), the existence of acarbon-containing layer was confirmed, the carbon-containing layer beingconstituted of a portion extending outward in a fibrous form or afilament form about 1000 nm in length from the surface of the aluminumfoil and a large number of particle portions having a particle diameterof about 0.5 μm which particle portions were adhered onto the surface ofthe above portion. Also, the presence of aluminum carbide was confirmedby X-ray analysis and an electron energy loss spectrometer (EELS).

Example 3

2 Parts by weight of an aluminum powder having an average particlediameter of 1 μm was mixed with 1 part by weight of a vinyl chloridebinder and the mixture was dispersed in a solvent (toluene) to obtain acoating solution having a solid content of 30%. This coating solutionwas applied to both surfaces of an aluminum foil having a thickness of15 μm (JIS 1N30-H18) and dried. The thickness of the dried coating filmon one surface was 2 μm. This aluminum foil was kept at 620° C. for 10hours in a methane gas atmosphere. Then, when the surface of the samplewas observed by a scanning electron microscope (SEM), the existence of acarbon-containing layer was confirmed, the carbon-containing layer beingconstituted of a portion extending outward in a cactus form about 5000nm in length from a number of particle portions about 1 μm in particlediameter which particle portions were adhered onto the surface of thealuminum foil. This scanning electron microphotograph is shown in FIG.2. Also, the presence of aluminum carbide was confirmed by X-rayanalysis and an electron energy loss spectrometer (EELS).

Examples 4 to 10

2 Parts by weight of carbon black having an average particle diameter of0.1 μm and 2 parts by weight of an aluminum powder having an averageparticle diameter of 1 μm were mixed with 1 part by weight of a vinylchloride binder and the mixture was dispersed in a solvent (toluene) toobtain a coating solution having a solid content of 30%. This coatingsolution was applied to both surfaces of an aluminum foil having athickness of 12 μm (JIS A3003-H18) and dried. The thickness of the driedcoating film on one surface was 4 μm. This aluminum foil was subjectedto heat treatment under the conditions shown in Table 1. In Example 8,the aluminum foil was subjected to rolling treatment carried out using arolling roll at a reduction ratio of about 20% after the heat treatment.In Example 10, the aluminum foil was subjected to activating treatmentcarried out at 300° C. for 2 hours in the air after the heat treatment.Then, when the surface of the sample was observed by a scanning electronmicroscope (SEM), the existence of a carbon-containing layer wasconfirmed, the carbon-containing layer being constituted of a portionextending outward in a cactus form from a large number of particleportions having a particle diameter of about 1 μm which particlesportions were adhered onto the surface of the aluminum foil and a largenumber of particle portions having a particle diameter of about 0.1 μmwhich particle portions were adhered onto the surface of the aboveportion. The scanning electron microphotograph of Example 6 is shown inFIG. 3. Also, the presence of aluminum carbide was confirmed by X-rayanalysis and an electron energy loss spectrometer (EELS). TABLE 1 Heattreatment Atmosphere Temperature (° C.) Time (Hr) Example 4 Acetylenegas 440 60 Example 5 Acetylene gas 490 10 Example 6 Methane gas 540 10Example 7 Methane gas 590 10 Example 8 Methane gas 590 10 Example 9Methane gas 640 10 Example 10 Methane gas 540 10 (Reference Example)

2 Parts by weight of carbon black having an average particle diameter of0.1 μm and 2 parts by weight of an aluminum powder having an averageparticle diameter of 1 μm were mixed with 1 part by weight of a vinylchloride binder and the mixture was dispersed in a solvent (toluene) toobtain a coating solution having a solid content of 30%. This coatingsolution was applied to both surfaces of an aluminum foil having athickness of 12 μm (JIS A3003-H18) and dried. The thickness of the driedcoating film on one surface was 4 μm. This aluminum foil was dried at500° C. in an argon gas atmosphere. Then, when the surface of the samplewas observed by a scanning electron microscope (SEM), nocarbon-containing layer was formed on the surface of the aluminum foilin the condition that it was adhered onto the surface of the aluminumfoil, the portions where the coating film was separated and fallen downwere observed and the presence of a portion extending outward in afibrous form, a filament form or a cactus form from the surface was notconfirmed. Also, the presence of aluminum carbide was not confirmed byX-ray analysis and an electron energy loss spectrometer (EELS).

Conventional Example

An aluminum foil having a thickness of 40 μm (JIS A1080-H18) wassubjected to a.c. etching treatment carried out at 50° C. and a currentdensity of 0.4 A/cm² for 60 seconds in an electrolytic solutioncontaining 15% of hydrochloric acid and 0.5% of sulfuric acid and theetched aluminum foil was washed with water and dried.

The characteristics of the respective samples obtained in Examples 1 to10, the reference example and the conventional example were measured inthe following manner.

(1) Thickness

The thickness of each sample was measured on an optical and sectionalmicrophotograph taken at a magnification of 100. The results ofmeasurement are shown in Table 2.

(2) Capacitance

The capacitance of each sample was measured by a LCR meter in an aqueousammonium borate solution (8 g/L).

(3) Mechanical Strength

The mechanical strength of each sample was evaluated by a tensile test.Specifically, using an Instron type tensile tester, a sample having awidth of 15 mm was stretched at a tensile rate of 10 mm/min to calculatethe tensile strength of the sample from the obtained thickness of thesample.

The capacitance and tensile strength of each sample measured in theabove manner are expressed by indexes when the values of thesecharacteristics obtained in the conventional example were respectivelyset to 100. The results are shown in Table 2. TABLE 2 Thickness ofCapacitance Tensile strength sample (μm) (%) (%) Example 1 32 112 195Example 2 32 196 208 Example 3 18 155 212 Example 4 20 103 269 Example 520 109 254 Example 6 20 327 237 Example 7 20 948 197 Example 8 16 1030523 Example 9 20 914 180 Example 10 20 603 227 Reference example 20 11252 Conventional example 38 100 100

As is clarified from these results of measurement, the samples ofExamples 1 to 10 respectively have an enlarged surface area andtherefore have a higher capacitance and a higher tensile strength thanthe sample of the conventional example. The sample of the referenceexample has a higher tensile strength but a lower capacitance than thesample of the conventional example.

The above disclosed embodiments and examples are illustrative in allpoints and are not to be considered to be restrictive. The scope of thepresent invention is defined by the appended claims rather than by theabove embodiments and examples and all variations and modificationswithin the scope of the claims and within the meaning of equivalence areinvolved.

INDUSTRIAL APPLICABILITY

The foil for a cathode of a capacitor according to the present inventionis suitable for securing both a high capacitance and a high strength.

1. A foil for a cathode of a capacitor, comprising: an aluminum foil;and a carbon-containing layer formed on a surface of said aluminum foil,the foil further comprising: an interposition layer that is formedbetween said aluminum foil and said carbon-containing layer and containsaluminum and carbon.
 2. The foil for a cathode of a capacitor accordingto claim 1, wherein said carbon-containing layer includes therein aninterposition material containing aluminum and carbon.
 3. The foil for acathode of a capacitor according to claim 1, wherein saidcarbon-containing layer is formed so as to extend outward from thesurface of said aluminum foil.
 4. The foil for a cathode of a capacitoraccording to claim 1, wherein said interposition layer constitutes afirst surface portion that is formed on at least a part of the surfaceof said aluminum foil and contains a carbide of aluminum, and saidcarbon-containing layer constitutes a second surface portion that isformed so as to extend outward from said first surface portion.
 5. Amanufacturing method of a foil for a cathode of a capacitor, comprisingthe steps of: arranging an aluminum foil in a space containing ahydrocarbon-containing substance; and heating said aluminum foil.
 6. Themanufacturing method of a foil for a cathode of a capacitor according toclaim 5, wherein said step of arranging the aluminum foil involvesadhering at least one kind selected from the group consisting of acarbon-containing substance and an aluminum powder to a surface of thealuminum foil and, then, arranging the aluminum foil in a spacecontaining a hydrocarbon-containing substance.
 7. The manufacturingmethod of a foil for a cathode of a capacitor according to claim 5,wherein said step of heating the aluminum foil is carried out within atemperature range between 450° C. or more and less than 660° C.